In metal sorting devices, objects to be sorted are conveyed on a conveyer belt and sorted according to their electromagnetic properties. In order to determine their electromagnetic properties, the objects which are conveyed on the conveyer belt are scanned by an electromagnetic scanner or sensor. The electromagnetic sensor identifies metallic objects and a processor activates one or more air nozzles located downstream in a conveying direction to blow the metallic objects into a container which collects them. In case of the non-metallic objects, the respective air nozzles are not activated and the object falls down into a container for the non-metallic objects, when the objects reach the end of the conveyer belt.
In those metal sorting devices, the inventors discovered that the conveyer belt gets damaged over long-term usage, because of metallic dust, nails or scraps which get stuck in the belt over time. This damage may influence the sorting quality. For example, false detections could cause the air nozzles to blow on non-metallic objects, because the loose non-metallic object lays on a metallic contaminant which is stuck in the belt, which could lead to impurities in the sorting result.
A solution could be increasing a threshold value at which the air nozzles are activated for old conveyer belts. This way, small contaminants which are stuck in the conveyer belt, would not lead to an activation of the air nozzles. However, this would have the disadvantage that small metallic objects would not be detectable anymore.